1. Technical Field
The disclosure relates to a multi-layered sheet, a backsheet for a photovoltaic cell, a method of preparing the same and a photovoltaic module.
2. Related Art
Recently, there is growing concern about renewable, clean energy because of global environmental problems and exhaustion of fossil fuels. Among these, solar energy has attracted attention as a representative pollution-free energy source which can solve problems of environment contamination and exhaustion of fossil fuels.
A photovoltaic cell to which a principle of photovoltaic power is applied is a device for converting sunlight into electricity. Since the photovoltaic cell is necessarily exposed for a long time to the outdoor environment to facilitate absorption of sunlight, it is made into a unit by performing several packaging processes to protect the cell. The unit is referred to as a photovoltaic module.
Generally, the photovoltaic module uses a backsheet having excellent weather resistance and durability to stably protect the photovoltaic cell during long-term exposure to the outdoor environment. The backsheet generally includes a backsheet in which a resin layer including a fluorine-based polymer such as polyvinyl fluoride (PVF) is stacked on a substrate.
However, because of its poor adhesion to a polyethylene terephthalate (PET) film typically used as a substrate for the backsheet, the PVF resin is used by laminating a fluorine-based polymer film obtained by extrusion or casting on a substrate using a urethane-based adhesive. However, for this technique, high-priced film preparation equipment, an adhesive and additional adhesive coating and laminating processes are needed. In addition, for this technique, it is necessary to use a thicker film than is required in order to handle the film, the use of various additives and fillers are limited, and a high processing temperature is required.
On the other hand, when a fluorine-based polymer film, which is prepared in the form of resin suspension or solution, is coated and dried on a substrate, a solvent having generally a high boiling point is used and a high drying temperature of 200° C. or more is needed.
A large amount of energy is used to provide a high drying temperature required by the PVF resin solution. This increases the production cost of the photovoltaic module backsheet and causes thermal shock or thermal deformation of the substrate. Consequently, the quality (mechanical characteristics, etc.) of a product is degraded, and its mechanical properties rapidly deteriorate over long-term outdoor use.
Therefore, there is continuing demand for a material for a photovoltaic cell backsheet which has excellent durability and weather resistance and can be dried at a low temperature, thereby reducing its production cost and enhancing the productivity and quality of a photovoltaic module.